It depends on what the search is about.
There are search terms that simply do not work as on or two word queries if you want to get back meaningful results.
Take
cars for instance. Everyone knows searching for that phrase by itself is going to return millions of sites with relevancy to hundreds of different aspects of the subject. Even adding the word Ford to the search doesn't give the kind of relevancy that most searchers are looking for and even adding adding a third word to the term still may not get the results that someone in Peoria is looking for when they want to shop for a locally available Ford Taurus.
SEO's should understand that commercial traffic for this type of searcher will only come from at least three word phrases and thus should be targeting them from the start.
So IMO while the statistics are interesting, you still have to optimize for what works without consideration of the number of words in the phrase